This invention relates to a battery separator for use in flooded cell type lead acid batteries. The separator has a shoulder design that provides improved resistance to puncture.
Separators are used in lead acid batteries to separate the positive and negative plates. Such separators are formed of materials that have sufficient porosity to permit the battery""s electrolyte to reside in the pores of the material, thereby permitting ionic current to be established between adjacent positive and negative plates, but not so porous as to allow physical contact between the plates or xe2x80x9ctreeingxe2x80x9d of lead between adjacent plates.
The most commonly used material in flooded cell type lead acid batteries is microporous polyethylene. Such separators and their method of manufacture are, essentially, described in U.S. Pat. No. 3,351,495.
Typically such separators have multiple xe2x80x9cmajorxe2x80x9d ribs formed on at least that planar face of the xe2x80x9cbackwebxe2x80x9d which is to face the positive plate. Such major ribs are typically formed parallel to the longitudinal edges of the backweb and typically have a height above the backweb that is greater than the thickness of the backweb.
Although at one time separators were manufactured as sheets which were placed between the plates, the overwhelming configuration currently used is to wrap the separator material around either the negative or positive plates and seal the separators, which has a width greater than the wrapped plates, along both edges to thereby form an xe2x80x9cenvelopexe2x80x9d around each wrapped plate. The separator edges adjacent the plate edges, where sealing of the separator is effected, is known as the xe2x80x9cshoulderxe2x80x9d area of the separator.
U.S. Pat. No. 4,407,063 discloses the most commonly used method and apparatus for enveloping and sealing separators around plates for lead acid batteries.
One method of making plates for lead acid batteries is to cut, slit, and expand a lead sheet. Such xe2x80x9cexpanded metalxe2x80x9d plates often have sharp metal protrusions, called xe2x80x9cbent wiresxe2x80x9d, which can abrade and puncture the adjacent separator shoulder thereby causing a short circuit and premature failure of the battery. Such punctures can occur during enveloping, during battery assembly, or after manufacture of the battery.
One approach to preventing such punctures is to make the shoulder area thicker than the thickness of the adjacent backweb. This approach is disclosed in U.S. Pat. No. 4,788,113.
Another approach has been to form xe2x80x9cmini-ribsxe2x80x9d in the shoulder area. Such mini-ribs have a cross-section that is the arc of a circle. The height of such mini-ribs above the backweb is less than that of the major ribs, typically 0.007 inch. Such prior art mini-ribs are also spaced closer together than the major ribs, a typical such mini-rib spacing being 0.0626 inch. The use of mini-ribs is preferable over using a thicker shoulder area because of cost considerations. However, it has been found that many punctures still occur in the shoulder area with separators using current mini-rib configurations.
It is an object of the present invention to provide a separator having an improved shoulder design that greatly reduces puncture by bent wires.
The present invention is a battery separator having a shoulder design that has improved puncture resistance.
The separator of the invention has a plurality of mini-ribs extending from one planar face of the separator in the shoulder area, the mini-ribs having a flattened upper surface rather than the rounded upper surfaces of prior art mini-ribs. The maximum spacing between the mini-ribs of the invention is 0.0225 inch.
In another embodiment of the invention, a plurality of micro-ribs extend from the planar surface of the separator opposite the planar surface from which the mini-ribs extend.